Why is module.exports check needed ?

[leopsidom]

I'm trying to understand this piece of code:

if (typeof module !== 'undefined' && module.exports) {
    module.exports = Prism;
}

here: https://github.com/PrismJS/prism/blob/master/prism.js#L1209

The reason I'm looking at this is because sometimes, when I run import { languages } from 'prismjs', I got languages as undefined. And when I print out module.exports, it gives me undefined:

console.log('module: ', module, 'module.exports: ', module.exports)
if (typeof module !== 'undefined' && module.exports) {
    console.log('module.exports!')
    module.exports = Prism;
}

module: { exports: [Getter/Setter] } module.exports: undefined

It's weird though that exports seems to exist in module, but it still module.exports is undefined.

Can someone help understand why the check is needed ? And why can sometimes the module.exports be undefined ?

Thanks